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Molecular Twisting Affects the Solid‐State Photochemical Reactions of Unsaturated Ketones and the Photomechanical Effects of Molecular Crystals

The three group of chalcone derivatives and their analogues involving halogen atoms (X = F, Cl, Br) have been synthesized. Firstly, the nearly planar acyclic chalcone derivatives were inclined to undergo photo-induced stereospecific [2+2] cycloaddition, which triggered the crystals to exhibit macroscopic motions of bending or cracking. In particular, the single-crystal-to-single-crystal transformation happened upon UV irradiation of their crystals, which was helpful for the understanding photomechanical effects. The group of cyclic 3,4-dihydronaphthalen-based chalcone analogues possess a more twisted conformation, and they tend to take place trans-cis isomerization. No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate. The twist degree of chroman-based chalcone analogues molecules was in between of the first two, [2+2] cycloaddition and trans-cis isomerization simultaneously underwent in their crystals. Photo-induced bending and twisting were observed for the crystals of chroman-based chalcone analogues. Therefore, the differences in molecular dihedral angles in a,b-unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects. In this work, a bridge between the molecular structures and solid-state photochemical reactions triggered photomechanical crystals is built.

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